The fuel cell is a device to generate electricity by electrochemical reaction between fuel and oxygen (oxidizing gas). It has recently come into the spotlight because it gives off only water after reaction without environmental pollution. Attempts are being made to use it as a power source to drive an automobile or as a household cogeneration system.
Active developmental works are under way to apply the fuel cell not only to the automotive power source etc. mentioned above but also to portable electronic equipment such as notebook personal computers, mobile phones, and PDAs (Personal Digital Assistant). The fuel cell for this purpose is required to constantly produce a rated power and have an adequate size and weight convenient for carrying. Technologies to meet this requirement are under development.
Some of recent electrical products have more functions and higher performance than before and hence they are provided with several electric circuits for individual functions, so that each electric circuit is driven at an adequate voltage. In the case of personal computer, for example, arithmetic elements are driven at about 1.65 to 3.3 V and electric circuits on the logic board are driven at about 5 V. Electronic components such as magnetic memory, which involve physical movement, works at 12 V, and liquid crystal display devices work at about 150 to 250 V. These electrical products are provided with a power source, such as AC adaptor, secondary battery, and fuel cell, to supply electric power at a prescribed voltage. They are also provided with a means to raise or lower the voltage of output from the power source so that individual electric circuits operate at an adequate voltage.
FIG. 8 shows an example of the conventional system in which a fuel cell is used as a power source for an electrical product having more than one electric circuit. In FIG. 8, the fuel cell 10 is of stack type composed of several cells placed one over another, so that layered cells are connected in series for generated current. The lowermost cell 11 is connected to the ground potential 12 and the uppermost cell 13 is connected to the electrode terminal 14.
The electrode terminal 14 is connected to the regulators to convert voltage which are arranged in parallel. The regulators 15, 16, 17, and 18 are connected respectively to a liquid crystal display, a circuit for the drive system with physical movement, a logic board, and an arithmetic element, so that they supply voltage at different levels suitable for individual circuits.
The regulators 15, 16, 17, and 18, which are connected to the electrode terminal 14, raise or lower the voltage supplied from the electrode terminal 14. The regulators receive the same voltage from the fuel cell 10 but their output voltage varies from one regulator to another. This means that the ratio of voltage increase or decrease greatly differs among the regulators 15, 16, 17, and 18.
In addition, the fuel cell usually greatly varies in voltage depending on load current. The disadvantage of supplying an output from the single electrode terminal 14 to various electric circuits is that the operation of one electric circuit affects other electric circuits connected thereto. This makes it difficult to minimize energy loss. The above-mentioned situation adversely affects individual elements, thereby reducing the power generating efficiency.
It is an object of the present invention to provide a fuel cell and a voltage supply method which are designed to supply power to various electric circuits in a stable manner.